Bug report: 1872168528
I was editing a document with Google Docs (with the Zotero plugin) when I launched Zotero and added a reference (https://www.scielosp.org/article/icse/2005.v9n18/621-630/) while Zotero was still loading my library and/or synchronizing it. Zotero throwed an error (obj.toJSON is not a function) and asked me to post the bug report.
If you can reproduce this, we'd want to see a Debug ID for a sync attempt that produces it.